1. Field of the Invention
The present invention relates to methods and apparatus for indicating the position of a core member of a variable differential transformer and more particularly to such methods and apparatus in which a signal is applied to the transformer primary and the secondary signal is analyzed to determine the core position.
2. Setting of the Invention
Variable differential transformers may be used to detect the position of a moving element of interest. A linear variable differential transformer is used to detect linear position and a rotary variable differential transformer is used to detect rotary position.
A linear variable differential transformer includes a primary winding and one or more secondary windings. When the movable transformer core is centered or in a null position, there is substantially a zero output on the secondary when a sinusoidal voltage is applied to the primary. When the core is moved in one position away from the null point the voltage on the secondary increases proportional to core displacement with the secondary signal being in phase with the primary signal. When the core is moved from the null position in the other direction, the secondary voltage again increases proportional to displacement; however, the signal is 180.degree. out of phase with the primary signal. Thus, the secondary signal includes information from which the position of the transformer core can be determined.
When the core is connected to an element, for example, a rapidly vibrating piece of machinery or a valve which is rapidly actuated, positional information of interest may be determined by demodulating the secondary signal of the linear variable differential transformer.
Prior art systems uniformly apply analog signals to the primary input. Some such circuits simply filter out the AC component of the secondary signal to generate an analog signal proportional to core position. Such heavy filtering removes information which could be used to more accurately detect transformer core position.
Other prior art circuits use the alternating current primary signal to trigger a sample and hold circut which periodically detects the value on the transformer secondary coil. Such circuits use zero-cross detection of the primary signal in order to generate a periodic signal which initiates sampling of the secondary signal. Compared to digital triggering on the edges of a digital wave form, analog zero-cross detection circuits cannot repeatably locate the point at which the primary wave form passes through zero This problem induces significant error when dealing with low frequency primary wave forms.